"Franken's service begins on the date on which he is sworn. So regardless of state size, etc, he cannot jump over members who were sworn on Jan. 6th," Reid spokesman Jim Manley said in an email. "So Mr. Franken is Number 100 and is last in seniority for all members and, of course last, for Democrats too."

Seniority determines where a senator sits on a committee, particularly in terms of committee leadership. It can also affect seemingly smaller matters, such as what desk a senator has in the chamber and the odds of getting a better office when there is a vacancy. Sen. Robert Byrd (D-W.V.) is the chamber's highest-ranking senator and serves as president pro tempore. That position allows him to preside over the Senate and puts him in line for the presidency.